An anchoring element in the form of a monoaxial bone screw is described in U.S. Pat. No. 5,005,562 and in the form of a polyaxial bone screw is described in DE 43 07 576 C1. Typically such anchoring elements comprise a receiving part for the reception of a rod connecting several anchoring elements of this type, a bone anchoring screw element that is monoaxially or polyaxially connected to the receiving part, and an internal screw that is to be screwed in between the legs of the receiving part and serves for the fixation of the rod. The receiving part according to DE 43 07 576 C1 further comprises an external screw nut that can be screwed onto the receiving part for additional fixation.
In such anchoring elements, the internal thread or external thread of the receiving part has incorporated a thread runout. Further, the distance between the rod support within the receiving part and the thread runout is smaller than the diameter of the rod to be received. This ensures that the internal screw or the external screw nut can exert a sufficient clamping force on the rod.
For a given rod diameter, the length of the free legs of the receiving part and, thus, the overall height of the anchoring element from the rod support generally is determined by the distance over which the internal and/or external thread is provided to ensure reliable fixation. In the area of the thread runout, which may include more than one thread tooth, the depth of the thread diminishes and the profile of the thread tooth is poorly defined. In order to provide for sufficiently firm attachment of the internal screw or external screw nut, it must be ensured that a sufficient number of turns of thread with a deep profile are present. Therefore, it is necessary either to place the thread runout sufficiently far down in the direction of the rod support or a corresponding number of turns of thread with a deep profile must be provided upwards in the direction of the free end of the legs which increases the overall height.
Moreover, the manufacture of the thread with a thread runout is resource-consuming from the point of view of manufacturing technology.
Thus, it is desirable to have an improved anchoring element of the type described above which is easy to manufacture and has a reduced overall height, but provides the same degree of safety against the rod becoming loose as is provided by the conventional anchoring elements.